Lithographic apparatus and device manufacturing method

ABSTRACT

In an immersion lithography apparatus, ultrasonic waves are used to atomize liquid on a surface of the substrate.

This application is a continuation of U.S. patent application Ser. No.10/986,186, filed Nov. 12, 2004, now U.S. Pat. No. 7,414,699 which isincorporated herein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. In,general, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent application WO99/49504, hereby incorporated in its entirety by reference. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletIN onto the substrate, preferably along the direction of movement of thesubstrate relative to the final element, and is removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

In a lithographic apparatus that confines immersion liquid, such aswater, to only a localized area between the projection system and thesubstrate, problems may be caused by any liquid left behind on thesubstrate after the projection system and liquid supply system havepassed (from the point of view of the substrate). For example, liquidleft on the substrate surface may evaporate causing localized cooling ofthe substrate which can lead to thermal distortion of the substrate.Humid gas resulting from such evaporation may affect the results ofinterferometric displacement measuring systems commonly used to monitorthe position of the substrate table.

SUMMARY

Accordingly, it would be advantageous, for example, to provide anapproach to removal of residual liquid from the substrate in animmersion lithography apparatus.

According to an aspect of the invention, there is provided alithographic projection apparatus arranged to project a pattern from apatterning device onto a substrate through a liquid provided in a spaceadjacent the substrate, the apparatus comprising an ultrasonictransducer configured to emit an ultrasonic beam toward the substrate toatomize liquid thereon.

According to an aspect of the invention, there is provided alithographic projection apparatus, comprising:

an illuminator configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning deviceconfigured to impart the radiation beam with a pattern in itscross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate;

a liquid supply system configured to at least partly fill a spacebetween the projection system and the substrate with a liquid, theliquid supply system providing the liquid onto the substrate; and

an ultrasonic transducer configured to emit an ultrasonic beam towardthe substrate to atomize liquid thereon.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising:

projecting a patterned beam of radiation onto a substrate through aliquid provided in a space adjacent the substrate; and

projecting an ultrasonic beam toward the substrate to atomize liquidthereon.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system for use in a lithographicprojection apparatus; and

FIG. 6 depicts a liquid supply system according to an embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam PB (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PL configured to project a pattern imparted to the radiation        beam PB by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam PB is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam PB passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam PB. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam PB, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. Such a system is shown inFIG. 5. The liquid confinement structure is substantially stationaryrelative to the projection system in the XY plane though there may besome relative movement in the Z direction (in the direction of theoptical axis). A seal is formed between the liquid confinement structureand the surface of the substrate. In an embodiment, the seal is acontactless seal such as a gas seal. Such a system with a gas seal isdisclosed in U.S. patent application Ser. No. 10/705,783, herebyincorporated in its entirety by reference.

FIG. 5 depicts an arrangement of a reservoir 10, which forms acontactless seal to the substrate around the image field of theprojection system so that liquid is confined to fill a space between thesubstrate surface and the final element of the projection system. Aliquid confinement structure 12 positioned below and surrounding thefinal element of the projection system PL forms the reservoir. Liquid isbrought into the space below the projection system and within the liquidconfinement structure 12. The liquid confinement structure 12 extends alittle above the final element of the projection system and the liquidlevel rises above the final element so that a buffer of liquid isprovided. The liquid confinement structure 12 has an inner peripherythat at the upper end preferably closely conforms to the shape of theprojection system or the final element thereof and may, e.g., be round.At the bottom, the inner periphery closely conforms to the shape of theimage field, e.g., rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the liquid confinement structure 12 and the surface of thesubstrate W. The gas seal is formed by gas, e.g. air, synthetic air, N₂or an inert gas, provided under pressure via inlet 15 to the gap betweenliquid confinement structure 12 and substrate and extracted via firstoutlet 14. The overpressure on the gas inlet 15, vacuum level on thefirst outlet 14 and geometry of the gap are arranged so that there is ahigh-velocity gas flow inwards that confines the liquid. It will beunderstood by the person skilled in the art that other types of sealcould be used to contain the liquid.

FIG. 6 shows a liquid supply system IH according to an embodiment of theinvention. The liquid supply system comprises a liquid confinementstructure 22 which confines liquid 11 to a space between the finalelement of the projection system PL and the substrate W. The liquidconfinement structure 22 is borne a small distance, e.g. 50 to 300 μm,above the substrate and has a seal device 23 to restrict outflow of theliquid 11. This may be a gas or liquid seal, using a flow of gas orliquid to confine the liquid 11, and may also act as a bearing for theliquid confinement structure which alternatively may be separatelysupported and/or actuated. The seal device may simply be a low pressureextraction port to suck away liquid flowing under the liquid confinementstructure 22.

Since it is typically difficult to make the seal device perfectlyeffective, it is likely that a thin film of liquid 11 a, perhaps of theorder of 10 μm thick, will be left behind as the substrate moves (e.g.,scans) in the direction of the arrow S. (The relative height of thisfilm is exaggerated in FIG. 6 for clarity). To remove this film, anultrasonic transducer 24 is provided on the lower surface of the liquidconfinement structure 22.

The ultrasonic transducer 24 emits an ultrasonic beam 25 with afrequency, in an embodiment, in excess of 1 MHz and potentially as highas 50 MHz through the gas (e.g., air) present under the liquidconfinement structure 22 and onto the substrate. The ultrasonic beam 25atomizes the liquid layer 11 a and frees the liquid from the surface ofthe substrate. The liquid is removed substantially without cooling ofthe substrate, by atomization and transportation of the atomized liquid.Evaporation, as does occur, will likely not directly affect thesubstrate. A flow of gas (e.g. air) 27 may be provided by gas supply 26to carry the atomized liquid toward the seal device 23 and prevent itfrom escaping to parts of the apparatus where it may be undesirable. Anextractive part of the seal device 23 may set up sufficient gas flow toperform this function in one or more embodiments of the invention.

In an embodiment, the ultrasonic transducer 24 is formed by twoseparately driven parts 24 a, 24 b. The relative phases of these partsmay be controlled to focus the ultrasonic beam at the surface of theliquid layer 11 a. A sensor 28, e.g. a an acoustic or optical sensor,may be provided to detect the presence of liquid layer 11 a and/or theposition of its top surface. Additionally or alternatively, the sensor28 may be used to control the amplitude, frequency and/or phase of thetwo parts 24 a, 24 b of the ultrasonic transducer to ensure efficientand effective removal of the liquid layer without excessive heatgeneration and to control the size of the liquid droplets generated,which may be a function of frequency of the ultrasonic beam.Interference effects between beams emitted by the two parts of thetransducer may be exploited to enhance atomization. Harmonics of theprincipal frequency may be used to enhance the removal effect. Althoughseveral tens of Watts of power may be input to the transducer per cm² ofsubstrate where liquid is to be removed, the frequency of the ultrasonicbeam(s) is sufficiently high not to disturb the lithographic apparatus.

Suitable types of transducers include piezo-electric, piezo-strictive,magneto-strictive and capacitive transducers. Multiple transducers maybe spaced around the liquid confinement structure 22 according to itsgeometry and the expected directions of movement of the substrate.Interdigitated transducers may be used and several concentric rings oftransducers may be used if required.

In place of a sensor to detect the liquid layer 11 a, the impedance ofthe transducer(s) may be monitored as proper coupling of the ultrasonicbeam 25 into the liquid layer 11 a will cause a change of impedance inthe transducer 24.

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the present invention may be applied to anyimmersion lithography apparatus, in particular, but not exclusively, tothose types mentioned above. A liquid supply system is any mechanismthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise any combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets, thecombination providing and confining the liquid to the space. In anembodiment, a surface of the space may be limited to a portion of thesubstrate and/or substrate table, a surface of the space may completelycover a surface of the substrate and/or substrate table, or the spacemay envelop the substrate and/or substrate table.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic projection apparatus, comprising: a substrate tableconstructed to hold a substrate; a projection system configured toproject a patterned radiation beam onto a target portion of thesubstrate; a liquid supply system configured to at least partly fill aspace between the projection system and the substrate with a liquid, theliquid supply system comprising a liquid confinement structureconfigured to at least partly confine liquid in the space; and anultrasonic transducer configured to emit an ultrasonic beam to removeliquid, the ultrasonic transducer located in or on the liquidconfinement structure and the ultrasonic beam emitting surface of theultrasonic transducer is non-perpendicular to a top surface of thesubstrate and/or substrate table.
 2. The apparatus according to claim 1,wherein the ultrasonic transducer is configured to emit the ultrasonicbeam into a gaseous medium.
 3. The apparatus according to claim 1,wherein the ultrasonic transducer is configured to emit ultrasonic beamsthat interfere at or near a surface.
 4. The apparatus according to claim1, comprising two ultrasonic transducers spaced apart in a directionsubstantially parallel to a periphery of the space.
 5. The apparatusaccording to claim 1, comprising two ultrasonic transducers spaced apartin a direction substantially radial to the space.
 6. The apparatusaccording to claim 1, wherein the ultrasonic transducer is configured toemit an ultrasonic beam having a frequency in the range of 1 to 50 MHz.7. The apparatus according to claim 1, further comprising: a sensorconfigured to detect liquid on a surface; and a control systemconfigured to control the amplitude, frequency, phase, or anycombination of the foregoing, of the ultrasonic beam in response to anoutput of the sensor.
 8. The apparatus according to claim 1, furthercomprising: a sensor configured to detect an impedance of thetransducer; and a control system configured to control the amplitude,frequency, phase, or any combination of the foregoing, of the ultrasonicbeam in response to an output of the sensor.
 9. A lithographicprojection apparatus, comprising: a substrate table constructed to holda substrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate; a liquid supplysystem configured to at least partly fill a space between the projectionsystem and the substrate with a liquid; and a sensor configured todetect, additionally or alternatively to the presence of a liquid layer,a location of a top surface of the liquid layer on a surface of thesubstrate, or of the substrate table, or of both the substrate and thesubstrate table.
 10. The apparatus according to claim 9, wherein thesensor comprises an acoustic sensor.
 11. The apparatus according toclaim 9, wherein the sensor comprises an optical sensor.
 12. Theapparatus according to claim 9, further comprising an ultrasonictransducer configured to emit an ultrasonic beam.
 13. The apparatusaccording to claim 12, wherein the ultrasonic transducer is part of aliquid confinement structure configured to at least partly confineliquid in the space.
 14. The apparatus according to claim 12, whereinthe ultrasonic transducer is configured to emit ultrasonic beams thatinterfere at or near a surface.
 15. The apparatus according to claim 12,wherein the ultrasonic transducer is configured to emit an ultrasonicbeam having a frequency in the range of 1 to 50MHz.
 16. The apparatusaccording to claim 12, further comprising a control system configured tocontrol the amplitude, frequency, phase, or any combination of theforegoing, of the ultrasonic beam in response to an output of thesensor.
 17. A device manufacturing method, comprising: projecting apatterned beam of radiation onto a substrate through a liquid providedin a space adjacent the substrate; at least partly confining the liquidin the space using a liquid confinement structure; and projecting anultrasonic beam toward the substrate to remove liquid using anultrasonic transducer located in or on the liquid confinement structure,wherein the ultrasonic beam emitting surface of the ultrasonictransducer is non-perpendicular to a top surface of the substrate. 18.The method according to claim 17, comprising projecting ultrasonic beamsthat interfere at or near a surface.
 19. The method according to claim17, wherein the ultrasonic beam has a frequency in the range of 1 to 50MHz.
 20. A device manufacturing method, comprising: projecting apatterned beam of radiation onto a substrate through a liquid providedin a space adjacent the substrate held by a substrate table; anddetecting, additionally or alternatively to the presence of a liquidlayer, a location of a top surface of the liquid layer on a surface ofthe substrate, or of the substrate table, or of both the substrate andthe substrate table.
 21. The method according to claim 20, furthercomprising controlling the amplitude, frequency, phase, or anycombination of the foregoing, of an ultrasonic beam in response to thedetecting of the liquid.